Treatment of carbon fibers to improve shear strength in composites

ABSTRACT

The interlaminar shear strength of carbon fiber-plastic composites is improved by treating the fibers, before they are incorporated into the plastic matrix, with an aqueous solution of hypochlorous acid having a pH below 6.

United States Patent [191 Barr [ TREATMENT OF CARBON FIBERS TO IMPROVE SHEAR STRENGTH IN COMPOSITES [75] Inventor: John Baldwin Barr, Strongsville,

Ohio

[73] Assignee: Union Carbide Corporation, New

York, N,Y.

[22] Filed: Oct. 21, 1970 [21] Appl. No.: 82,816

[52] U.S. Cl. 106/307, 423/447 [51] Int. Cl. C09c l/44 [58] Field of Search 106/307; 23/209] F, 209.9

[56] References Cited UNITED STATES PATENTS 3,660,140 5/1972 Scola et al 23/209.l F

[451 Feb. 12,1974

3,476,703 11/1969 Wadsworth ct al 106/307 3,330,799 7/1967 Voet 106/307 2,199,936 5/1940 Kauffman .1 23/152 OTHER PUBLICATIONS Miyamichi et al., Chemical Abstracts, Vol. 64, 1966, Col. 12,862 (b)(c).

Primary Examiner-James E. Poer Attorney. Agent, or Firm-John S. Piscitello [57] ABSTRACT 51 Claims, N0 Drawings BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to improved composite articles comprising a plastic matrix reinforced with carbon fibers.

2. Description of the Prior Art Composites consisting of a plastic matrix reinforced with carbon fibers are of interest in applications where materials of high strength-and high modulus-to-weight ratios are required, e.g., in aircraft structures, reentry vehicles, space vehicles, and marine deep-submergence pressure vessels. A limitation on the use of such composites, however, is the relatively low bond strength of such fibers to the plastic matrix.

It has been suggested that the strength of the bond between the fibers and the matrix may be improved by treating the fibers to modify their surface by a process such as oxidation. Among the oxidizing agents proposed is sodium hypochlorite [Goan, 1C. and Prosen, S.P., lnterfacial Bonding in Graphite Fiber Resin Composites, American Society for Testing and Materials, lnterfaces in Composites, ASTM, Philadelphia, Pa., 1969, pp 3-26, (ASTM pub. 452)]. However, only modest composite shear strengths have been achieved by such treatment except when very lengthy soaking times are employed, such as 2 or 3 days, or when the treatment is effected in a heated solution. Extended treating periods are undesirable commercially, however, and treatmentwith heated sodium hypochlorite solutions, even for very limited times, causes serious degradation of the tensile strength of the fibers, rendering them unsuitable for use in preparing composites. The torsional shearstrengths of composites prepared from graphite fibers treated with an aqueous solution of sodium hypochlorite under varying conditions of time and temperatureare shown in Table I below, together with the Youngs modulus and tensile strength of the fibers so treated.

TABLE I Effect of Sodium Hypochlorite (5.25 wt. on Graphite Yarn. Composites Fabricated from Graphite Yarn (50vol.%) and an Epoxy Average of2 samples "Avcruge of 5 samples SUMMARY OF THE INVENTION In accordance with the instant invention it has now been discovered that the interlaminar shear strength of carbon fiber-plastic composites can be materially improved by treating the fibers, before they are incorporated into the plastic matrix, with an aqueous solution of hypochlorous acid having a pH below 6. The improvement in shear strength obtained by such treatment is shown in Table 11 below, together with the Youngs modulus and tensile strength of the graphite fibers treated in this manner.

TABLE II Effect of Hypochlorous Acid on Graphite Yarn. Composites Fabricated from Graphite Yarn (50vol.%) and an Epoxy Resin Prepared by acidifying a 5.25 wt.% aqueous solution of sodium hypochlorite to a pH of 4 to 5 with chlorine "Average 012 samples '"Average of 5 samples As can be seen from a comparison of Tables 1 and 11, significantly higher shear strengths can be obtained by treating the fibers at a given time and temperature with hypochlorous acid solution than can be obtained with sodium hypochlorite solution, or, alternatively, comparable shear strengths can be obtained in much shorter soaking times at a given temperature with hypochlorous acid solution than can be obtained with sodium hypochlorite solution. If still further reductions in soaking times are desired, a soluble chloride salt may be added to the solution to raise the chloride ion concentration. The shear strength and tensile strength values obtained by treating fibers under varying conditions of time and temperature with hypochlorous acid solutions to which a soluble chloride salt has been added is shown in Table III below.

TABLE III Treatment of Graphite Yarn with Hypochlorous Acid-Sodium Chloride Solution. Composites Fabricated from Graphite Yarn (50 vol. and an Epoxy Resin TABLE III-Continued Treatment of Graphite Yarn with Hypochlorous Acid-Sodium Chloride Solution". Composites Fabricated from Graphite Yarn (50 As can be seen from a comparison of Tables 11 and 111, it is possible to obtain comparable shear strength in 'much shorter soaking times at a given temperature by the addition of a soluble chloride salt to the hypochlorous acid solution, or alternatively, significantly higher shear strengths can be obtained by treating the fibers at a given time and temperature with a hypochlorous acid solution to which a soluble chloride salt has beenadded than can be obtained from a hypochlorous acid solution to which a soluble chloride salt has not been added. Thus, while several hours soaking is required at room temperature to obtain a composite shear strength in excess of 8 X 10 psi. when achloride salt has not been added to the solution, a treating time of as little as 1 hour is often sufficient at room temperature to obtain such shear strength when using solutions to which a chloride salt has been added. As can be further seen from Tables II and III, the tensile strength of fibers at such shear strength levels is comparable whether or not,

sodium chloride has been added to the solution.

In contrast to the result obtained when a soluble chloride salt is added to a hypochlorous acid solution, it is not possible to shorten the soaking time necessary to obtain comparable shear strength by adding a soluble chloride salt to a solution of sodium hypochlorite unless high treating temperatures are employed. This is demonstrated by a comparison of Table I above with Table IV below.

TABLEIV Treatment of Graphite Yarn with Sodium Hypochlorite-Sodium Chloride Solution". Composites Fabricated from Graphite Yarn (50 {TABLE IV-Continued Treatment of Graphite Yarn with Sodium Hypochlorite-Sodium Chloride Solution, Composites Fabricated from Graphite Yarn (50 vol. and an Epoxy Resin Strand Tensile Strength. l0 psi Reaction Conditions Temp. C, Time, Hours Prepared by adding 200 grams of sodium chloride per liter of a commercially available bleach solution having a pH of about 11 and containing 5.25 wt. sodium hypochlorite and 4.8 wt. 1* sodium chloride "Average of 2 samples 'Average of 5 samples DESCRIPTION OF THE PREFERRED EMBODIMENTS Because of the unstablenature of hypochlorous acid, the aqueous, solution employed to treat the carbon fibers is prepared immediately prior to use, preferably by acidifying a water solution of a soluble hypochlorite salt, such as sodium hypochlorite, potassium hypochlorite, lithium hypochlorite, calcium hypochlorite, and the like, to a pH value of 6 or less, preferably to between 4 to 5. In the case of more stable hypochlorite salts, such as calcium hypochlorite, the solution can be conveniently prepared by dissolving the solid hypochlorite salt in water. In the case of hypochlorite salts which are unstable in their solid state, such as sodium hypochlorite, it is necessary to prepare the aqueous solution by chemical reaction in water. By way of illustration, an aqueous solution of sodium hypochlorite can be produced by the reaction of chlorine with a water solution of sodium hydroxide according to the equatron:

2NaOH C1 NaOCl NaCl 11 0 After the water solution of the hypochlorite salt has been prepared, it is converted to hypochlorous acid by acidfying the solution to a pH value. of 6 or less, preferably to between 4 to 5. Acidification of the solution is preferably effected by the addition of hydrochloric acid or chlorine, but any acidifying agent which will cause a lowering of the pH to the desired value can be employed. The hypochlorite salt should be present in the solution in an amount which would provide the stoichiometric amount of hypochlorite ion necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion and the hypochlorite ion were fully converted to hypochlorous acid. Preferably, the water-soluble salt is employed in an amount which would provide an excess of hypochlorite ion of at least 2 times, most preferably from 3 to 5 times, above that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion. The reaction by which sodium hypochlorite is converted to hypochlorous acid by chlorine and hydrochloric acid can be illustrated by the equations:

NaOCl C1 0 NaCl 2HOCl NaOCl HCl l-1 NaCl HOCl The conversion of calcium hypochlorite to hypochlorous acid can be illustrated by the equation: I

Ca (0C1) 2HC1 g Ca Cl 2HClO Suitable hypochlorous acid solutions can be prepared according to the invention by adding hydrochloric acid or chlorine to a solution containing at least 1.2 grams, preferably from 2 grams to 6 grams, of sodium hypochlorite per 100 grams of water until the desired pH is obtained. When calcium hypochlorite is employed, at least 4.8 grams of the calcium hypochlorite salt is dissolved per 100 grams of water before the addition of hydrochloric acid or chlorine to the solution to adjust the pH.

If a water soluble chloride salt is added to the solution to reduce the soaking time required, it should be added in an amount which together with other chloride ions present (e.g., from the hydrochloric acid or chlorine added to acidify the solution or the chloride produced by adding chlorine to convert sodium hydroxide to sodium hypochlorite) is sufficient to raise the total chloride ion concentration to a level of at least 1.5 gram-atoms per liter, preferably from 3.2 gram-atoms per liter to 4.7 gram-atoms per liter. Suitable watersoluble chloride salts which can be employed include solution before it is acidified. As long as the solution remains basic it is stable and can be stored for long periods of time, but once it is acidified to convert the hypochlorite to hypochlorous acid, the solution becomes unstable and must be used immediately.

After the hypochlorous acid solution has been prepared, the carbon fibers are immersed therein and allowed to soak until a fiber which will give the desired composite shear strength has been obtained. For convenience, the fibers may be wrapped around a spool or similar object before being immersed in the acid solution. The duration of the soaking period depends upon how great a composite shear strength is desired, as well as upon'tensile strength requirements and the temperature of the hypochlorous acid bath. While the composite shear strength increases with increased soaking times, the tensile strength of the fibers is degraded as soaking continues. For this reason, prolonged soaking periods are undesirable and should be limited to prevent excessive degradation of the tensile strength of the fibers. Likewise, while heating serves to accelerate the increase in shear strength, it also causes more rapid degradation of the tensile strength of the fibers, so that excessive heating is also undesirable.

in order to form useful carbon fiber plastic matrix composites a minimum shear strength of 8 X 10 psi. together with a minimum fiber tensile strength of 2 X 10 psi. is generally required. To obtain such shear strengths, it is usually necessary to soak the fibers in the hypochlorous acid solution for a period of about 7 hours at 30 C., or about 5 'hours at C. or 3 hours at 70 C. When a soluble chloride salt is added to the solution, soaking times of about 3 hours are usually required at 30 C., while about 30 minutes are required at 50 C., and about 10 minutes are necessary at 70 C. In order for the fiber to also have the desired tensile strength, the temperature and/or soaking time should be limited and the starting fibers employed should have a sufficiently high tensile strength so that they are not degraded below 2 X 10 psi. by the hypochlorous acid solution under the conditions employed. In the case of carbon fibers having a tensile strength before treatment of about 250,000 psi.,'the desired final tensile strength can be obtained, in the case when a soluble chloride salt has been added to the solution, by limiting soaking to about 7 hours at 30 C., or 30 minutes at 50 C., or

10 minutes at C. Preferably, the hypochlorous acid bath is maintained at from about 20 C. to 30 C. whether or not a soluble chloride salt is added to the solution. However, equally good results can be obtained at higher temperatures by limiting the soaking time.

After the carbon fiber has been immersed in the hypochlorous acid bath for a satisfactory length of time, it is removed from the bath, washed with water and dried. The fiber may then be compounded with a plastic matrix in accordance with known techniques.

High modulus high strength carbon fibers suitable for use in the instant invention can be prepared as described in US. Pat. Nos. 3,503,708 and 3,412,062.

Any thermosetting resin binder which is compatible with carbon or graphite yarn can be employed as the matrix in producing the carbon fiber-resin composites of the present invention. Suitable resins include, by way of illustration, phenolic resins, epoxy resins, Friedel-Crafts resins, and the like.

Carbon fiber-resin composites are usually fabricated by coating the fibers with a suitable resin binder and, subsequently, curing the resin binder after the article to be constructed has been formed to shape to produce a rigid fiber-resin composite or structure.

The following example is set forth for purposes of illustration so that those skilled in the art may better understand this invention, and it should be understood that it is not to be construed as limiting this invention in any manner. The term carbon as used throughout this specification includes all forms of the material, both graphitic and non-graphitic.

EXAMPLE 1 Samples from nine different lots of two-ply graphite yarn were separately wrapped on nine Pyrex glass spools 3.5 inches high and 12 inches in circumference. The yarn contained 720 filaments per ply with the filaments characterized by an average Young's modulus of 50,000,000 psi. and an average tensile strength of 288,000 psi. The spools were designed to allow free access of a treating bath to the graphite fiber when sub merged in the bath.

The spools were placed in glass vessels which were filled with hypochlorous acid solution to a level which covered the spools when they were placed in the solution, and the vessels were covered. The hypochlorous acid solution was prepared by acidifying a commercially available bleach solution having a pH of about ll and containing 5.25 wt.-% sodium hypochlorite and 4.8 wt.-% sodium chloride to a pH of 4 to 5 by slowly bubbling chlorine through the solution. The chlorine was introduced into the reaction vessel through a glass tube submerged in the solution, allowed to bubble slowly through the solution, and then conducted into a concentrated sodium carbonate bath to remove unreacted chlorine. For convenience, the flow of chlorine was usually continued for the duration of the experiment. The bath was continuously stirred with a Teflon coated bar and a magnetic stirrer. After soaking for 7 hours at room temperature, the spools were removed from the bath, washed for about 1 hour in flowing distilled water, and dried by heating for 1 hour at a temperature of C.

The procedure was then repeated twice employing a bath prepared by adding 200 grams of sodium chloride per liter of a commercially available bleach solution No. 3-Treated at 70C for 15 minutes in asolutionpreparedinggirtfingwith Nb? 2. n 7

having a pH of about 11 and containing 5.25 wt.-% 50- 0.34 moles per liter if the hypochlorite radical were dium hypochlorite and 4.8 wt.-% sodium chloride, and fully converted to hypochlorous acid. then acidifying the solution to a pH of4 to with chlo- 2. A process as in Claim 1 wherein the hypochlorous rine in the manner described above. In both instances acid Solution has a PH of from 4 to nine samples,of l yam were employed as above 5 3. A process as in claim 2 wherein the hypochlorous and the treating conditions were the same except that acid Solution has a temperature of from to in the first instance the spools were, soaked for 3 hours C instead of 7, and in the second instance they were soaked for 0.25 hours and the bath was maintained at 70 C. instead of room temperature.

The yarns prepared in this manner were tested for tensile strength and fabricated into unidirectional composites with an epoxy resin (ERL 225 6, a commercially available liquidepoxy resin, was employed together with 19 pph metaphenylene diamine as a hardening agent). The composites contained 50 volume percent fibers. The amount of fibers required to produce such composites was predetermined from the cross-sectional area of the fibers and the volume of the mold employed. The correct amount of fiber was wound on a plastic spool having a circumference of about 7 inches and then impregnated under vacuum at room tempera- I ture with the epoxy resin containing the hardener. The A P Q Clalm 5 where!" the hypochlorous impregnated yarn was then cut from the spool and acld solution has a P of Q 4 to placed unidirectionally in a steel mold. The yarn length .25 8. A process as in claim 6 wherein the hypochlorous was trimmed to the size of the mold and the ends of the acid solution has a temperature of from 20 C. to 30 mold were sealed with Teflon strips. Pressure was then C. applied evenly and the resin was cured by heating at 9. A process as in claim 7 wherein the hypochlorous 80 C. for 3 hours and then at 160 C. for 2 hours. The acid Solution has a emperature of from 20 C. to 30 cured composite was then removed from the mold and 30 C 4. A process in claim 1 wherein the hypochlorous acid solution contains an excess of hypochlorite radical of at least 2 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the hypochlorite radical were fully converted to hypochlorous acid. I

5. A process as in claim 4 wherein the hypochlorous acid solution contains an excess of hypochlorite radical of from 3 to 5 times that necessary toproduce a hypochlorous acid concentration of 0.34 moles per liter if the hypochlorite radical were fully converted to hypochlorous acid.

6. A process as in claim 4 wherein the hypochlorous acid solution has a pH of from 4 to 5.

machine ground to-a cylinder 5 inches long and 0.150 A Process as in claim 4 wherein the hypochlorous inches in diameter The Shear Strength f tha composite acid solution also contains a chloride ion concentration was determined by the torsional method as described of at least 15 gram-atoms P men by Adams, DR, and Thomas, R.L., The Solid-Rod 11. A process as in claim 10 wherein the hypochlo- Torsion Test for the Determination of Unidirectional 5 rous acid solution has a pH offrom 4 to 5. Composite Shear Properties, Textile Research Jour- 12, A ro e as in l i 11 h rei th h hl nal, 339-345 (April, 1969). rous acid solution has atemperature of from 20 C. to

The tensile strength of the fibers and the torsional 30 C, shear strength of the composites are set forth in Table 13, A process as i l i 5 h i th h hl V below. acid solution contains a chloride ion concentration of TABLE V Treatment ofGraphite Yarn in Hypochlorous Acid and Hypochlorous Acid-Sodium Chloride Solutions. Composites Fabricated from Graphite Yarn vol. and an Epoxy Resin Sample Torsional Shear Strength,l0psi Strand Tensile Strength, lo psi As Rec'd No.1 No.2 No.3 As Recd No.1 No.2 No.3

1 5.9 7.6 8.8 9.7 262 242 210 195 2 5.3 10.1 9.7 9.4 247 212 216 187 3 5.3 11.7 8.8 9.7 277 244 292 261 4 4.9 6.9 6.8 8.1 302 264 289 303 5 4.4 9.1 7.9 6.1 278 268 270 256 6 5.6 7.3 8.0 8.7 314 289 309 316 7. 4.3 8.1 8.2 7.4 297 285 351 296 8 4.5 6.6 8.3 11.3 326 268 269 325 9 4.4 6.8 5.8 6.5 292 276 321 295 avg. 5.0 8.2 8.0 8.5 288 261 281 270 No. l Treated at room temperature for 7 hours in a solution prepared by acidifying a commercially available bleach solution having a pH of about 11 and containing 5.25 wt. sodium hypochlorite and 4.8 wt. sodium chloride to a pH of 4 to 5.

No, 2-Treated at room temperature for 3 hours in a solution prepared by adding 200 grams of sodium chloride perjt er of a commercially 'available ble'ach solution having a pH of about 1 1 and containing 5.25 wt. sodium hypochlorite and 4.8 wt. %sodium chloride, and then acidifying the solution to a pH of 4 to 5.

What is claimed is: from 3.2 gram-atoms per liter to 4.7 gram-atoms per 1. A process of treating high modulus high density liter. carbon fiber to improve the bonding characteristics of 14. A process as in claim 13 wherein the hypochlosaid fiber to a plastic matrix comprising soaking the rous acid solution has a pH of from 4 to 5.

fiber in an aqueous solution of hypochlorous acid hav 15. A process as in claim 14 wherein the'solution has ing a pH below 6, said solution containing at least the a temperature of from 20 C. to 30 C.

stoichiometric amount of hypochlorite radical neces- 16- A process of treating high modulus high density sary to produce a hypochlorous acid concentration? carbon fiber to improve the bonding characteristics of tion and the hypochlorite ion were fully converted to hypochlorous acid.

17. A process as in claim 16 wherein the hypochlorous acid solution has a pH of from 4 to 5.

18. A process as in claim 17 wherein the hypochlorous acid solution has a temperature of from 20 C. to 30 C.

19. A process as in claim 16 wherein the hypochlorite salt is present in the solution in an amount which would provide an excess of hypochlorite ion of at least 2 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion.

20. A process as in claim 19 wherein the hypochlorite salt is present in the solution in an amount which would provide an excess of hypochlorite ion of from 3 to 5 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion.

21. A process as in claim 19 wherein the hypochlorous acid solution has a pH of from 4 to 5.

22. A process as in claim wherein the hypochlorous acid solution has a pH of from 4 to 5.

23. A process as in claim 21 wherein the hypochlorous acid solution has a temperature of from 20 C. to 30 C.

24. A process as in claim 22 wherein the hypochlorous acid solution has a temperature of from 20 C to 30 C 25. A process as in claim 16 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

26. A process as in claim 17 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

27. A process as in claim 18 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

28. A process as in claim 19 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

29. A process as in claim 20 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

30. A process as in claim 21 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifyv ing agent is selected from the group consisting of chlorine and hydrochloric acid.

31. A process as in claim 22 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

32. A process as in claim 23 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidify ing agent is selected from the group consisting of chlorine and hydrochloric acid.

33. A process as in claim 24 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

34. A process as in claim 16 wherein. the water solution of soluble hypochloric salt also contains a soluble chloride salt and said soluble chloride salt is present in said solution in an amount which together with the other chloride ions present is sufficient to raise the total chloride ion concentration of the solution to a level of at least 1.5 gram-atoms per liter.

35. A process as in claim 34 wherein the hypochlorous acid solution has a pH of from 4 to 5.

36.,A process as in claim 35 wherein the hypochlorous acid solution has a temperature of from 20 C. to 30 C.

37. A process as in claim 34 wherein the hypochlorite salt is present in the solution in an amount which would provide an excess of hypochlorite ion of at least 2 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion.

38. A process as in claim 37 wherein the hypochlorite salt is present in the solution in an amount which would provide an excess of hypochlorite ion of from 3 to 5 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion, and the chloride salt is present in said solution in an amount which together with other chloride ions present is sufficient to raise the total chloride ion concentration of the solution to a level of from 3.2 gram-atoms per literto 4.7 gram-atoms per liter.

39. A process as in claim 37 wherein the hypochlorous acid solution has a pH of from 4 to 5.

40. A process as in claim 38 wherein the hypochlorous acid solution has a pH of from 4 to 5.

41. A process as in claim 39 wherein the hypochlorous acid solution has a temperature of from 20 C. to 30 C. i

42. A process as in claim 40 wherein the hypochlorous acid solution has a temperature of from 20 C. to 30 C.

43. A process as in claim 34 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

. 44. A process as in claim 35 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

45. A process as in claim 36 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

46. A process as in claim 37 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

47. A process as in claim 38 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

48. A process as in claim 39 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.

49. A process as in claim 40 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying and hydrochloric acid. 

2. A process as in claim 1 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 3. A process as in claim 2 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 4. A process in claim 1 wherein the hypochlorous acid solution contains an excess of hypochlorite radical of at least 2 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the hypochlorite radical were fully converted to hypochlorous acid.
 5. A process as in claim 4 wherein the hypochlorous acid solution contains an excess of hypochlorite radical of from 3 to 5 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the hypochlorite radical were fully converted to hypochlorous acid.
 6. A process as in claim 4 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 7. A process as in claim 5 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 8. A process as in claim 6 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 9. A process as in claim 7 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 10. A process as in claim 4 wherein the hypochlorous acid solution also contains a chloride ion concentration of at least 1.5 gram-atoms per liter.
 11. A process as in claim 10 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 12. A process as in claim 11 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 13. A process as in claim 5 wherein the hypochlorous acid solution contains a chloride ion concentration of from 3.2 gram-atoms per liter to 4.7 gram-atoms per liter.
 14. A process as in claim 13 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 15. A process as in claim 14 wherein the solution has a temperature of from 20* C. to 30* C.
 16. A process of treating high modulus high density carbon fiber to improve the bonding characteristics of said fiber to a plastic matrix comprising (1) preparing a water solution of a soluble hypochlorite salt, (2) acidifying the solution to a pH below 6, and (3) soaking the fiber in the acidified solution, said hypochlorite salt being present in said solution in an amount which would provide at least the stoichiometric amount of hypochlorite ion necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion and the hypochlorite ion were fully converted to hypOchlorous acid.
 17. A process as in claim 16 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 18. A process as in claim 17 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 19. A process as in claim 16 wherein the hypochlorite salt is present in the solution in an amount which would provide an excess of hypochlorite ion of at least 2 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion.
 20. A process as in claim 19 wherein the hypochlorite salt is present in the solution in an amount which would provide an excess of hypochlorite ion of from 3 to 5 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion.
 21. A process as in claim 19 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 22. A process as in claim 20 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 23. A process as in claim 21 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 24. A process as in claim 22 wherein the hypochlorous acid solution has a temperature of from 20* C to 30* C.
 25. A process as in claim 16 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 26. A process as in claim 17 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 27. A process as in claim 18 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 28. A process as in claim 19 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 29. A process as in claim 20 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 30. A process as in claim 21 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 31. A process as in claim 22 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 32. A process as in claim 23 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 33. A process as in claim 24 wherein the soluble hypochlorite salt is sodium hypochlorite and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 34. A process as in claim 16 wherein the water solution of soluble hypochloric salt also contains a soluble chloride salt and said soluble chloride salt is present in said solution in an amount which together with the other chloride ions present is sufficient to raise the total chloride ion concentration of the solution to a level of at least 1.5 gram-atoms per liter.
 35. A process as in claim 34 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 36. A process as in claim 35 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 37. A process as in claim 34 wherein the hypochlorite salt is present in the solution in an amount which would provide an excess of hypochlorite ion of at least 2 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion.
 38. A process as in claim 37 wherein the hypochlorite salt is present in the solution in an amount which would provide an excess of hypochlorite ion of from 3 to 5 times that necessary to produce a hypochlorous acid concentration of 0.34 moles per liter if the reaction caused by the acidification proceeded to completion, and the chloride salt is present in said solution in an amount which together with other chloride ions present is sufficient to raise the total chloride ion concentration of the solution to a level of from 3.2 gram-atoms per liter to 4.7 gram-atoms per liter.
 39. A process as in claim 37 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 40. A process as in claim 38 wherein the hypochlorous acid solution has a pH of from 4 to
 5. 41. A process as in claim 39 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 42. A process as in claim 40 wherein the hypochlorous acid solution has a temperature of from 20* C. to 30* C.
 43. A process as in claim 34 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 44. A process as in claim 35 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 45. A process as in claim 36 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 46. A process as in claim 37 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 47. A process as in claim 38 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 48. A process as in claim 39 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 49. A process as in claim 40 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 50. A process as in claim 41 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid.
 51. A process as in claim 42 wherein the soluble hypochlorite salt is sodium hypochlorite, the soluble chloride salt is sodium chloride, and the acidifying agent is selected from the group consisting of chlorine and hydrochloric acid. 